1. Field of the Invention
The present invention relates, in general, to streak cameras or tubes and, more specifically, to sweep drive or sweep generator circuits for streak cameras.
2. Description of the Art:
Optical transient events in the picosecond range, such as events generated by use of ultra fast pulsed lasers, can be measured using streak cameras or tubes. The streak camera includes an electron tube containing a photocathode, a phosphor screen, electron optics consisting of several focusing electrodes, and at least two deflection electrodes or plates.
In a common mode of operation, the investigated optical phenomenon or event is typically in the form of a narrow optical line on the photo cathode, which produces a line image on the phosphor screen along the Y axis. A d.c. voltage applied to the deflection plates moves this line along the X axis perpendicular to the Y axis. To study the optical intensity as a function of time, an electrical signal in the form of a ramp is applied to the deflection plates. The end result is to spread the line image in the X direction or axis to form a streak on the screen. Under these conditions, the variation along the X axis of the optical intensity on the screen at a given level Y represents the time variation of the incoming optical signal at that particular Y level.
To detect very weak repetitive signals, it is necessary to average successive images. However, since the ramp signal is generated electronically, random fluctuations in the starting instant of the ramp will occur. For the necessary high voltage ramp signals, such variations are of the order of 10 picoseconds or more in state of the art streak camera systems. This severely limits the effective time resolution in the averaging mode when optical events are occurring in the sub ten picosecond range.
Mourou has proposed in U.S. Pat. No. 4,413,178 to generate the ramp signal synchronously with the optical signal by use of a photoconductor. In a typical implementation, the ramp generator includes two sections of transmission line and a photoconductor switch consisting of two electrodes on a semiconducting material with high resistivity in the dark or absence of light. The first transmission line is connected to the deflection electrodes through the switch and the second transmission line. A reference optical signal or pulse closes the switch transmitting the deflection voltage to the deflection or sweep electrodes.
Under ideal conditions in which the reference optical signal is perfectly synchronized with the optical phenomenon under study and there is negligible electrical resistance of the photo conductor switch in its conducting state as well as an absence of fluctuations of the high voltage signal, the ramp signal will be perfectly synchronized and averaging may be performed without loss of time resolution. However, it is difficult to provide perfect synchronization due to the finite value of the photoconductor switch resistance and the existence of fluctuations in the intensity of the synchronizing optical signal. These variations cause fluctuations in the amplitude of the ramp signal which result in fluctuations of the correspondence between the time instance of various positions along the X axis on the screen. At the start of the ramp signal, the ramp is stable at a point of time. Any voltage point along the ramp signal can fluctuate with respect to time. Unfortunately, in general, the slope of the ramp has a maximum in the middle of the ramp signal range. Thus, the constraints of a large ramp signal slope for a large time dependent deflection and low jitter are contradictory. As a consequence, averaging of the optical event cannot be performed with adequate temporal resolution.
Thus, it would be desirable to provide a sweep generator circuit for use in a streak camera which has a stable operating point with reduced fluctuation while maintaining large time dependent deflection.